In order to fulfill current stringent emission legislation more or less all vehicles with internal combustion engines are provided with an exhaust gas aftertreatment device comprising at least one catalytic converter with at least one catalytic substrate. A catalytic converter substrate generally comprises a channeled structure which exhaust gases can pass through while being exposed to the large surface area of the catalytic substrate. The channels of the substrates may be fluidly connected by perforating holes or like allowing gases to pass between adjacent channels. This enables gas to diffuse through the substrate structure. For petrol engines the most frequently used catalytic converters are of Three Way Catalyst (TWC) type, while catalytic converters of Diesel Oxidation Catalyst (DOC) type and/or Lean NOx Trap (LNT) type are the most frequently used converters for diesel engines. The TWC or the DOC/LNT may be supplemented by a converter with Selective Catalytic Reduction (SCR) functionality for improved NOx reduction. Typically, when using a catalytic converter of SCR type a liquid or gaseous reductant is added to the exhaust gas emission flow before the exhaust gases enters the catalytic converter of SCR type. The addition of reductant enables the catalytic reduction were NOx is reduced to diatomic nitrogen, N2, and water, H2O.
Catalytic converters combining the functionalities of more than one type of catalytic converter in one catalytic converter also exist.
Combining more than one catalytic converter can be problematic since exhaust gas aftertreatment devices often are associated with design restrains due to the limited available space in the engine compartment. Thus, small exhaust gas aftertreatment devices are preferred from an engine packaging perspective, but small exhaust gas aftertreatment devices usually means that the flow distance between the inlet and the catalytic substrates of the catalytic converter is limited. Limited distance means that the time and distance during which mixing of the exhaust gas emissions can occur is limited. Insufficient mixing of the exhaust gas emissions gives inhomogeneous exhaust gas emission mixture. This might e.g., be problematic for emission gas sensors, arranged in the exhaust gas emission flow, to work properly and give accurate emission measurements.
Other problematic areas for catalytic converters are high back pressure and insufficient heating. High back pressure implies significant exhaust gas flow resistance. This is negative for the efficiency of the combustion engine resulting in a decrease of power output. Compensation of such decrease in power output leads to an increase in fuel consumption. If there is a difference in back pressure between two possible flow paths the flow through the flow path with lowest backpressure will be larger than the flow through the flow path with the higher backpressure. The flow ratio will be in proportion to the difference in back pressure. Heating of the catalytic converter is crucial since the catalytic converter is most effective at relatively high temperatures. Thus, it is desirable that the catalytic converter reaches its optimum operation temperatures as soon as possible and that the catalytic converter stays warm during operation.
Insufficient mixing of the exhaust gas emissions are of particular interest if a catalytic converter with a catalytic substrate with SCR functionality is used. For catalytic converters with SCR functionality, a liquid or gaseous reductant such as e.g., urea is introduced in the exhaust gas emission flow in order for the reductant and exhaust gas emissions to mix before reaching the substrate with SCR functionality. When a liquid reductant is used it is also desirable that the liquid reductant is evaporated. Consequently, sufficient mixing and reductant evaporation is important for the substrate with SCR functionality to work properly.
U.S. Pat. No. 8,607,551 discloses an exhaust gas purifier and system for exhaust gas purification including an NOx catalyst of SCR type and a Catalyst Supported diesel particulate Filter (CSF) arranged in series, and being dispensed in an exhaust passage of an internal combustion engine. The purifier includes a passage for urea supply having a hydrolysis catalyst therein and a passage for hydrocarbon supply having an oxidation catalyst therein. Prior art, described in FIG. 4, discloses a DOC, a CSF and a catalyst of SCR type is arranged in series. A urea injection valve is arranged in a pipe upstream of the catalyst of SCR type. Neither in U.S. Pat. No. 8,607,551 nor prior art therein provides sufficient mixing and reductant (urea) evaporation characteristics and respective design are limited in regards of engine compartment packaging requirements.
Thus, there is a need for further improvements.